1. Field of the Invention
The present invention relates to poly-crystalline thin film transistors and to their fabrication. More particularly, the present invention relates to poly-crystalline thin film transistors having multiple section gate electrodes and multiple section semiconductor layers.
2. Description of the Background Art
Thin film transistors used in active matrix liquid crystal displays are typically made of amorphous silicon (a-Si). Amorphous silicon is advantageous in that it enables large area displays that can be fabricated at low temperature on low-cost glass substrates. However, since the mobility of a-Si thin film transistors is low, such transistors are not well suited for LCD drive applications. In contrast, poly-crystalline silicon thin film transistors have high mobility. This makes them interesting candidates for both LCD array and driving circuits.
FIG. 1 illustrates a poly-crystalline (p-Si) silicon transistor that is suitable for driving a pixel of a liquid crystal display. The transistor in FIG. 1 is a poly-crystalline silicon, CMOS (Complimentary Metal Oxide Semiconductor) thin film transistor that includes a buffer layer 3 on a transparent glass substrate 1. On the buffer layer 3 is a semiconductor layer having a p-Si intrinsic channel layer 4, an LDD region 5 that is doped with an impurity at a low concentration, and source and drain regions 6 that are impurity doped at a high concentration. A gate insulation layer 9 is formed over the substrate 1, and a gate electrode 2 is on the gate insulation layer 9 and over the channel layer 4. An interlayer 13 is deposited over the gate electrode 2 and over the gate insulation layer 9. The interlayer 13 includes via holes that expose the source and drain regions 6. Source and drain electrodes 11 are then formed on the interlayer 13 and in the via holes so as to contact the source and drain regions 6. A passivation layer 15 is then deposited over the interlayer 13 and source and drain electrodes 11, and a drain contact hole is formed through the passivation layer 15. A pixel electrode 17 is then formed on the passivation layer 15 and in the drain contact hole so as to contact the drain electrode 11.
As mentioned above, poly-crystalline silicon has a high mobility. Thus, the switching time of the transistor shown in FIG. 1 is fast, which reduces signal delays. In addition, p-Si driving circuits that use such CMOS thin film transistors can be formed along with pixel driving thin film transistors on the same liquid crystal panel substrate. This simplifies the fabrication processes.
However, poly-crystalline silicon thin film transistors constructed as shown in FIG. 1 tend to have higher leakage currents than amorphous silicon thin film transistor switching devices. High leakage causes the voltage applied to a pixel to drift, which can result in flicker and poor picture quality in a liquid crystal display.
As is well known, leakage current in a poly-crystalline silicon thin film transistor is related to the electric field distribution between the drain and source regions and the gate. While the poly-crystalline silicon thin film transistor illustrated in FIG. 1 reduces leakage current by incorporating the LDD region 5, which reduces the electric field along the sides of the source and drain regions, such poly-crystalline silicon thin film transistors have relatively high parasitic resistances and a relatively low ON-state current.